Opal, a curious gem, is composed of tiny silica spheres stacked on top of one another, a structure that creates a unique splash of rainbow colors. Found in sedimentary rock layers that lie beneath desert sands, opal ranges from transparent to opaque and can occur with or without the trademark flashes of color.

For most people, though, opal is synonymous with shimmering color, and the term “opalescence” describes a quality of iridescent rainbow hues. Of all the varieties of opal, the most desirable is a black-body-colored gem with glistening colored highlights. And the place to find it is Australia.

History and romance. The origins of opal lie more than 65 million years in the past, in the era of the dinosaurs. Opals are found in Australia’s deserts, which once were inland seas lined with silica beaches. When the seas disappeared, silica-filled groundwater seeped into cracks, clay layers, and fossils, then solidified and transformed into the slivers, pockets, and other forms of opal that exist today.

Shakespeare declared opal to be “the queen of gems.” White Czechoslovakian opal was a favorite of Queen Elizabeth I and thus became a trend throughout her court. Its prominence in England spawned the legend that wearing opal, October’s birthstone, is a ticket to wealth.

Color variations. Opal is a sedimentary rock, an aggregate of layer upon layer of uniform silica spheres. Light bouncing through the spheres is diffracted—broken into spectral hues. The smaller spheres produce purples, violets, blues, and greens. The larger spheres create yellows, oranges, and reds.

Black opals have a background or base color of black, dark blue, dark green, or dark gray, the result of impurities (such as iron oxide) or the host ironstone matrix. Because of the base color, black opals are opaque when viewed from above. The spectral colors on the top of the stone stand out in relief from the dark background. Stones with red fire on black, in broad patterns, are the most valuable.

Qualities. Gem-quality opals are found all over the world, but more than 90% come from Australia. Only a few countries produce black opal, including Australia, Brazil, Czechoslovakia, Honduras, Mexico, and the United States (Idaho and Nevada).

Three Australian states are important to opal production. In New South Wales, an area called Lightning Ridge produces spectacular black opal—some say it’s the finest of all. To the north, in Queensland, black boulder opals are found in Winton and Quilpie. It’s debatable whether these are of lesser quality than Lightning Ridge blacks, but some feel that their ironstone matrix disqualifies them from the designation “true black opal.” South Australia is home to the famous Coober Pedy, Mintabie, and Andamooka mines. Andamooka and Mintabie produce blacks, but in very small quantities.

Lightning Ridge black opal commonly occurs as “nobbies”—small round balls or lumps. The rough usually has a light gray outer surface of “potch”—common opal—that is cut open to expose the black opal. Black boulder opal from Queensland is found only in ironstone, which gives the opal strength, durability, and a dark background.

Australian opals reportedly are superior to opals from Nevada or Mexico, which, unlike their Australian counterparts, can absorb liquids, making them susceptible to crazing (see the following section on bench precautions).

Value. Opal’s vibrant display of spectral colors makes it unlike any other gem, and top-quality specimens can command prices that rival those of fine diamonds. The characteristic play of color (the iridescent flashes that change as the viewing angle changes) against a dark background is what makes black opal so valuable. Size and color of individual flashes can vary, but the more intense and vibrant the colors and the more even they are as they “sweep” across a stone, the more valuable the stone is. Bold colors that “race” across the stone from end to end and side to side offer the most value, but hue and pattern also are important. Red flashes command the highest price.

The latest prices. Fine-quality black opals of less than 15 cts. that exhibit predominantly green-blue fire range from $600 to $1,400 per carat. Opals of the same size and quality with a predominantly red-orange play of color range in price from $1,800 to $3,500 per carat.

Enhancements. Opals with white body color (which account for most of the world’s supply) can be darkened and made to look like black opals. Traditional enhancements include smoke treatment and sugar treatment. A newer treatment impregnates polymers into opal’s porous structure, creating specimens that mimic the finest-quality black opals. Only through magnification and infrared spectroscopy can these enhancements be detected.

Care and cleaning. Opal’s hardness ranks between 5 and 6.5 on the Mohs scale. Ammonia and other strong cleaning agents can ruin an opal’s surface polish. Clean these gems only with warm, soapy water, and never use steam or ultrasonic cleaners. Opals, like other soft gems, such as tanzanites and chrome diopsides, should be kept in jewelry pouches with individual pockets. Storing in dry, closed boxes, especially safe deposits, can damage opals. Opals contain water in their chemical structure, but they shouldn’t be soaked in water or oils. The back-and-forth transition from wet to dry can do more damage than leaving the gems alone would do. In fact, exposure to body oils and normal humidity is more likely to keep opals from drying out than soaking is.

Bench settings and precautions. Opals are easily scratched, so take great care when setting or repairing opal jewelry. Crazing, a surface and internal cracking, is common and unpredictable. Although it can occur at random, it usually happens when an opal goes from humid to dry conditions too quickly or when it’s exposed to intense light or heat—typical conditions in a jewelry showcase.

Beware of opal doublets and triplets (thin slices of precious opal typically glued onto a harder, more durable material), which are subject to damage during repair. Doublets are made of a thin layer of opal backed by black onyx or an ironstone compound. To create a triplet, a rock crystal quartz cabochon is placed atop a thin slice of opal, which is then backed. A black or dark material may be used to cement the base to the opal, creating a black-opal-like appearance.

Recommended reading. For more information on black opal, see the following:

A Journey with Colour: A History of Queensland Opal, 1869-1979 by Len Cram (Kingswood Press, Brisbane, 1998).

Opal Identification and Value by Paul B. Downing (Majestic Press, 1992).

Opals (Fred Ward Gem Book Series) by Fred Ward and Charlotte Ward (Gem Book Publishing, 1997).